Electric projection lamp



May 1968 R. E. LEVIN 3,381,575

ELECTRIC PROJECTION LAMP Filed Dec. 5, 1965 FILM GATE FIG. I AW OPTIC j/ m I AxIs 5W O LIMITING MERIDIAN PLANE RAYS ACCEPTED BY OBJECTIVE LENS E EKF AXES T SEMI-MAJOR: A

SEMI-MINOR:B

FILAMENT CROSS- -SECTION SHOWN FOR TWO MERIDIAN PLANES T 90 FILM GATE R IY OPTIC 'E AXIS ORIGIN OF COORDINATES FIG.2

ROBERT E. LEVIN INVENTOR.

I ATTORNEY United States Patent Oflice 3,381,575 Patented May 7, 1968 3,381,575 ELECTRIC PROJECTION LAMP Robert E. Levin, Hamilton, Mass, assiguor to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 3, 1965, Ser. No. 511,459 3 Claims. (Cl. 8824) ABSTRACT OF THE DISCLOSURE An incandescent filament of finite dimensions in a reflector, the filament and reflector being related for maximum effectiveness at a film gate and lens to conform with a mathematical formula.

This invention relates to the combination of an electric light source and reflector for supplying luminous flux to the filmgate and objective lens of a projection system. The combination is matched to the characteristics of the filmgate and objective lens in order to provide a pre-specified optimum performance. In particular the invention relates to an electric light source inside a sealed glass envelope having a blow-shaped portion whose inside surface carries a reflecting coating, for example of metal, but the invention is not limited to that specific embodiment.

However, a lamp of that type had not heretofore been efficiently used to illuminate a filmgate and objective lens because no suitable combination of reflector and filament was available. In most attampts, the lamp filament would be placed at one focus of an ellipsoidal reflector and the filmgate at the other, the filament being considered as a point source of zero dimensions, an assumption which cannot lead to the proper results. I have discovered that the filament can be taken as of finite dimensions, and a reflector and filament combination designed so that each cooperates with the other for maximum effectiveness at the film gate and lens.

By my invention, the proper combination of reflector and filament size, shape, and position can be determined by calculation and used in a lamp, or the reflector can be outside the lamp.

A device according to my invention can be an integral reflector and incandescent filament lamp, although the reflector and source may be separate devices if desired, and other sources, such as gaseous discharge lamps, can be used instead of the incandescent filament, if the luminous portion is of the proper dimensions.

The basic form of my reflector is an ellipsoid of revolution; an ellipsoid provides a reflector which concentrates energy from one small region of space to another relatively small region of space, these regions being generally called the focal points. However, with a filament of finite size the light cannot actually be focused from the first focal point to the second focal point because an ellipsoidal reflector is highly comatic. The invention imposes no requirement that either the source or any other part of the system be at a focal point. In general, the design does not place any physical combination nor any defined entity such as an entrance pupil at a focal point of the ellipsoid. The invention provides the lowest power means that fits within an arbitrary enclosure and provides essentially complete filling of the exit pupil.

The design for minimum power is equivalent to requiring the smallest physical filament. The actual filament as manufactured is made larger than the design value to allow for production tolerance. If the region of space representing the design filament is completely occupied by the manufactured filament, the designed performance will be obtained. The size difference between the manufactured filament and the designed filament represents the available production tolerance, if one is to insure that the manufactured filament at least fills the volume represented by the design filament. The additional filament volume allows some tolerance in positioning the lamp with respect to the filmgate.

Although the filament in the specific example below is taken as being cylindrical, that is, a coiled-coil in which the second coiling is circular and along a straight axis, the filament can be of oval or rectangular cross-section, instead of circular, if desired, and the mathematics adjusted correspondingly.

I have discovered that the design objectives are met when a cylindrical filament is used which intersects all rays traced back through the system after reflection from the reflecting surface; a few paraxial rays will be provided directly by the filament without reflection. Further, I have discovered the specified performance will be attained when the reflector shape and location plus filament size and location are designed to produce the smallest cylindrical surface which intersects all of the designated rays. In concept, a single set of equations can be formulated to specify the reflector and filament combination being discussed. The constants in these equations specify the size limitation on the lamp and the characteristics of the objective lens and filmgate combination. However, these equations are too unwieldly to apply directly for design purposes or even for a clear concise explanation. The description shall therefore be present in terms of parametric equations applied sequentially. The equations can be solved by either graphical techniques or on a digital computer in this sequential parametric form. The ellipsoidal reflector is extremely comatic; consequently analysis of an ellipsoidal reflector in terms of a point source of light gives erroneous results. The treatment of a finite source properly located for the specified performance makes my device unique.

Other objects advantages and features of the invention will be apparent from the following specification in which:

FIGURE 1 is a schematic diagram showing the path of I rays through a filmgate;

FIGURE 2 is a schematic drawing of a reflector, filament and filmgate; and

FIGURE 3 is a perspective view of a lamp according to the invention.

The lamp disclosed here is for use with a circularly symmetrical objective lens and a rectangular filmgate. Equations are formulated for meridian planes parallel to two adjacent sides of the filmgate. The basic filament shape is cylindrical, although it is made practicall with a coiled-coil of wire. A single coiling of wire is essentially the same optically and is considered to be equivalent under this invention. The first equations represent the characteristics of the filmgate and objective lens combination for which the lamp is designed.

pmax( 0+ 1 l 2 'l lt u and 13 are the limiting meridian plane angles of ray acceptance by the filmgate and objective lens combination as a function of radial distance, H, from the optic axis (see FIG. 1). Due to the circular symmetry of the lens, these equations are valid for all meridian planes out to the H limit imposed by the filmgate in any meridian plane. If a coordinate system is established as shown in FIG. 2, E P p and F are the limits imposed on the physical size of the lamp. The input parameters have now been established.

To express the particular configuration of the reflector and filament of this invention consider the following equa- Each ray through a point of the filmgate at radial distance H and angle Ot(-fi m strikes the reflector at (x y by expressing the ellipse in hyperbolic parametric form:

x =KS cos aE-|A (5) y =H+S sin 06 S k cos a H Silla cos he, sin ha,

2 sin a cos a (k sin a-i-H cos a) [(sin h cos hn sin h cos h sin a 2 cos a 2 l:( 1) +(00S n) :l

on? s =arc cos h VA -B 9) The reflected ray from (x y has an angle, 7, measured counter-clockwise from the negative abscissa, of

=arc sin 7 x S111 ha t-y cos M P is real for the ray intercepting the circle, and the minimum diameter circle occurs for equal roots, i.e.

The design of this invention requires that the filament size be minimized with respect to the four implicit variables representing refiector size and shape, reflector location, and filament location. Further, this minimum must occur within the parameter limits imposed. Finally, if the physical filament selected is not physically acceptable, the minimum area filament that is mechanically acceptable becomes the defined design. The mathematical solution for the size and location of the specific filament and the size, shape and location of the matched reflector of this invention can be based on Equations 1 through 12; many ditferent methods of obtaining the solution are possible.

As previously discussed an indirect solution is most expedient. The optimum filament for each reflector is determined over the range of possible reflectors. To examine a specific reflector, 0 consider that for any position of the filament there is a minimum circular crosssection that intercepts a given ray, '1' in the N meridan plane parallel to the short dimension of the filmgate. It is R (as determined by Equation 12) for the ray of index 1;, and R is a piece-wise linear function of F. Take a finite set of N rays through the filmgate in the N meridian plane selected with a relatively uniform spacing in ray space. For a set of H values within the bounds of the filmgate A two-dimensional array of R ,,(I-I) versus F determines the minimum radius at each F point to provide all required rays. Designate this function as R ,,(F); it is also a piecewise linear function of F. The analogue of this evaluation in the orthogonal meridian plane, M, determines the length, L (F) of the cylinder. Thus the cylindrical surface area T is known as a function of F for the reflector rl/ To scan the entire set of possible reflectors, W, the parameters are varied within the following restrictions:

and Equation 4 is also applied. Thus we obtain T (F, A, B, E). As a practical point note that the point (-P, must fall on the outside of the most extreme ray accepted by the filmgate and objective system if complete fill of the exit pupil is to be attained. Further, any other set of three ellipse parameters may be substituted for (A, B, E) depending on the numerical techniques chosen. Now by minimizing the function T (F, A, B, E), the specific values of R L F, A, B, and E are determined. In the case where (R L does not represent the bounds of a. mechanically desirable filament (R L is scanned as T (F, A, B, E) is allowed to depart from the minimum by AT. This process is continued until an acceptable (R L is attained. This point represents the minimum filament that is acceptable under all of the established criteria.

In FIG. 3 the lamp 1 has a sealed glass envelope 2, which includes a bowl-portion 3 and a cover portion 4. The inside surface 5 of the bowl-portion 3 carries a coating 6 of reflecting material 7, the surface 5 of the bowl and the coating thereon having the curve of an ellipsoid defined by the equations above. The coating can be of metal, but is preferably of a dichroic material such as that described in United States Patent 3,162,785, issued Dec. 22, 1964 to Scoledge et al. Such a material will refiect the visible light but allow the infra-red to be transmitted through it. In that way, only the visible light will reach the filmgate, allowing more light to be used without burning the film. The coating will generally be only a few thousandths of an inch thick.

Lead-in wires 9, 9 are sealed through the bowl portion 3 and support a coiled-coil filament 8 inside the bulb. The filament 8 is of tungsten wire coiled once, and then that coiled again as is customary in certain lamps. It can be considered as a cylinder. The bulb is filled with an inert gas at about an atmosphere pressure.

A specific lamp 1 embodying the invention is described below. It was designed for a super-8 mm. filmgate 0.159 in. x 0.213 in. with the filament 8 and reflector 6 to fit within a cylinder of radius 1.125 in. centered on the optic axis and within a distance range of 1.210 in. to 2.144 in. measured from the filmgate plane. The projection lens for use with the filmgate is an f/ 1.0, 22 mm. focal length projection lens. The design was made for focuse at infinity. By providing optical system fill for this condition, complete optical fill is also provided for all closer image distances. For this system the reflector 6 is a prolatc spheroid with semi-major and semi-minor axes A, B, of 1.5172 in. and 1.2600 in., respectively, with the apex located 2.1440 in. from the filmgate. The ellipsoid is cut off by a plane normal to the optical axis 0.8340 in. from the apex. The filament 8 is centered at 0.6990 in. from the apex and has a length and diameter of 0.140 in. and 0.075 in, respectively. This size includes an allowance for manufacturing and positioning tolerances of the filament. Tests of manufactured lamps showed that the performance was in complete accord with the theoretical development.

Although a particular embodiment is described above, various modifications will be apparent to a worker skilled in the art, without departing from the spirit and scope of the invention, which is set forth in the appended claims.

What I claim is:

1. The combination of an electric lamp, a film gate spaced therefrom and an objective lens spaced from both, said lamp comprising an approximately cylindrical light source of finite radius E and a finite length L and a substantially ellipsoidal reflector in reflecting relationship thereto and having a semi-major axis A and a semi-rninor axis B in a meridian plane, the center of the filament being at a distance F from the film gate, the apex of the reflector being at a distance E from the film gate, and the open circumference of the reflector being at a distance P from the film gate and having a radius the surface area of the light source being T the reflector curvature conforming to the following equations:

the latter equation being minimized for the smallest filament area T T being a piecewise linear function taken over the number of allpossible reflectors W' satisfying the conditions that:

UNITED STATES PATENTS 3,160,776 12/1964 Cardwell et a1. 3l3113 3,308,715 3/1967 Ashcraft 8824 3,314,331

4/1967 Wiley 8824 JAMES W. LAWRENCE, Primary Examiner.

R. L. JUDD, Assistant Examiner. 

